I think it's about having a batch of people you know have similar backgrounds around you rather than exploring on your own. At least to start. And oh yes, the big parties!!
And, as others put forth, it gives people a drop dead date if they are unable to break away on there own. If you are concerned about doing it on your own, there are others close by. And on some rallies, they do a lot of the clearance paperwork for the participants.

When I was looking at the Baja HaHa, they did not help on the paperwork. When I asked what I got for my $$, I was told about what great parties they had on the stops along the way. Heck, I can party on my own for a lot less $ than they were asking. So we went it on our own.

Greg